Valve drive mechanismThe valve drive mechanism commands the inlet and exhaust valves at the required timing.The valve drive mechanism consists of an inlet and exhaust cam follower [8] each suspended in a plunger of the tappet assembly [7], two tubular push rods [4] with ball joints, two nodular cast iron rocker arms [2] suspended in a rocker arm bearing bracket, two yokes (5气阀横臂/valve bridge/) and a yoke guide pins. See Fig.The tappet rollers follow the cam profiles and transfer the movements through push rods to the rocker arms. The rocker arms operate the inlet and exhaust valves through yokes [5].Lubrication for the rocker arms comes from the engine's secondary oil supply manifold through drillings in the cylinder head and rocker arm bracket.The tappet assembly receives lubricating oil via channels in the engine block from the main supply manifold. For the roller and shaft, the oil is supplied through the guiding plunger.To compensate for heat expansion a clearance must exist between the rocker arm and yoke. All adjustments have to be done on a cold engine only.

Which statement about the passage is not true?